1. Field of the Invention
The present invention relates to a process for producing granulated active carbon by carbonizing pellets of a styrene-divinyl benzene copolymer matrix with concentrated sulfuric acid at elevated temperatures.
2. Description of the Related Art
German Patent Application 196 00 237 describes a process for converting ion exchangers or their prestages, i.e., polymer pellets without functional groups, into high-value adsorbents on a carbon basis. Polymer pellets based on styrene and some divinyl benzene are especially suitable.
The process disclosed in German Patent Application 196 00 237 is as follows: Sulfuric acid is added at the beginning of the carbonization process. This prevents the polymer pellets from baking together and bursting, and results in a clear increase in yield. The sulfuric acid leads to sulfonization of the aromatic rings with formation of sulfonic (--SO.sub.3 H) groups and their decomposition with subsequent interlacing via radicals and sulfur bridges. The starting material can be in macroporous or gel-type form.
Because water is formed during sulfonization in accordance with the equatio n EQU H+HOSO.sub.2 OH.fwdarw.SO.sub.2 OH+H.sub.2 O
and this water would slow the reaction, it is important to continually remove the formed water. This can be done by means of a nitrogen flow, for example.
Complete sulfonization is important for various reasons. Incomplete sulfonization of the polymer pellets leads to inadequate interlacing, especially in the interior of the pellets, so that monomers could easily split off and gas outbreaks could occur, causing the outer shell of the pellets to burst.
Although bursting can be prevented by special temperature programs, products with low vibration density are nonetheless obtained, because complete carbonization and activation result in pellets that are very open-pored in the interior. Vibration weight is very important when a certain mass is to be located in a small space. For this reason, "light" pellets, i.e., products with low vibration density, are less in demand.
Complete sulfonization can be achieved using the process described in German Patent Application 196 00 237, especially when the upper limit of the stated range for the sulfuric acid quantity is approached. However, attention must be paid to many process parameters. For example, there must be a careful and relatively long temperature program as well as exactly dosed nitrogen rinsing, because it is only the moisture, not the sulfuric acid, that is to be removed. Furthermore, not all starting materials behave the same way. For example, anion exchangers can be sulfonized easily using this process, whereas prestages are much more difficult to sulfonize, regardless of whether they exist in macroporous or gel-type form.